Method for forming seamless pressure vessels



United Smtes Patent ABSTRACT OF THE DISCLOSURE Powders of metals such as beryllium, titanium, aluminum, molybdenum and tungsten are pressed and sintered to form a fully dense, powder metallurgy article which is subsequently deformed against a jacket in a hot working operation utilizing a fluid pressure applied to the unclad surface of the articles.

This invention relates in general to the fabrication of articles by powder metallurgy techniques and, more particularly, to the fabrication of seamless pressure vessels having containment walls characterized by a continuous, homogeneous, wrought microstructure.

Many of the high strength metals and alloys possess physical properties which would make their use attractive as the materials of construction for space age pressure vessels and similar hardware. Unfortunately, however, many of these materials are diflicult to fabricate by conventional methods and are, furthermore, susceptible to such physical phenomena such as localized embrittlement in or adjacent weld areas used to join one detail to another. As a consequence, articles formed of these materials have not enjoyed the wide utility or approached the potential which their theoretical capabilities would suggest, particularly in high stressed components such as pressure vessels.

Depending, of course, upon the particular ultimate application in mind and the environment to which the article is to be exposed, hardware of the nature hereinbefore discussed has been constructed at various times from beryllium, zirconium, titanium, the refractory metals including molybdenum and tungsten, and the dispersion strengthened nickel and aluminum alloys. While various fabrication techniques have been utilized, the powder metallurgy approach has shown particular promise, particularly in the manufacture of articles of irregular shape.

It is an object of the present invention to provide an improved method for fabricating seamless pressure vessels of high strength by powder metallurgy techniques.

A further object is to provide a method for producing powder metallurgy articles having containment walls characterized by a homogeneous wrought microstructure.

These and other objects and advantages will be discussed in the detailed description which follows or will be evident therefrom or from practice of the invention.

According to the present invention and briefly stated, the desired pressure vessels or similar articles are produced by: isostatically pressing metal powders of the desired composition into the approximate configuration of the finished article and sintering the same to form a unitary body having a density approaching the calculated theoretical density; cladding the sintered body with a jacket of high relative strength; hot working the pressure vessel in compression against the jacket by the application of a fluid pressure against the unclad wall surface; and subsequently removing at least the major portion of the jacket from the worked pressure vessel.

In connection with the initial pressing operation wherein the powders are first compacted, it will usually be found preferable to apply the requisite pressure to the powders Patented Dec. 24, 1968 'ice while they are held at a temperature near or above their sintering temperature in a hot pressing operation. In general, the simultaneous application of temperature and pressure will provide an article of the highest density and, usually, a fully dense article. The particular preferred pressures and temperatures used at this stage of the process will, of course, vary depending upon the particular composition of the materials, as will he recognized by those skilled in the art, and the sintering temperature will vary somewhat as an inverse function of the applied pressure. In any event, the temperature-pressure selection will be such as to yield a fully dense, sintered compact upon completion of the hot pressing operation. The particularly preferred pressing parameters for a number of suitable high strength materials are set forth in Table I.

TABLE I Material Temperature F.) Pressure (p.s.i.)

The simultaneous application of temperature and pressure in the pressing operation may not, in certain circumstances, be convenient or even feasible, due usually to equipment limitations. In such cases, the pressing and sintering phases will necessarily have to be performed sequentially. Satisfactory sintered bodies may be prepared by cold or warm pressing to the desired shape and, subsequently, subjecting the pressed powders to a sintering heat treatment.

In order to take full advantage of the strength increases associated with a wrought metallurgical grain structure, the powder metallurgy product is hot worked against a jacket or clad provided therearound. The jacket is applied to the article in any convenient manner and conforms in configuration to the surface of the article. The particular composition of the jacket is relatively unimportant but it will obviously possess the following properties: high strength relative to the article to be worked; thermal stability near or above the preferred hot working or recrystallization temperature of the material to be worked; preferably a different modulus of elasticity than the material to be worked; and chemical compatibility with the substrate. For the sake of economy, since the clad is expandable tooling which will subsequently be discarded, one of the more common and inexpensive alloys meeting the previously discussed qualifications is normally used. In some instances one further factor may influence the selection of the cladding material. It is preferable in many cases to chemically etch the jacket from the pressure vessel upon completion of the hot working operation and, accordingly, when such an operation is contemplated the material may advantageously be selected on the basis of its susceptibility to the etching process. In any event, the jacket will provide a surface against which the powder metallurgy product may be hot worked at the preferred working parameters.

The hot working operation is performed by applying a fluid pressure at the unclad wall surface of the vessel while it is held near or above the recrystallization temperature of the sintered material. To prevent contamination in the working operation, an inert medium such as argon or helium is preferred as the pressurizing fluid. The particular working temperatures and pressures utilized to produce the desired Wrought microstructure will vary according to the particular powder composition initially used. While the usual practice will involve establishment of the preferred hot working parameters prior to the selection of a jacket material of suitable composition and strength to withstand the working pressures, there is considerable flexibility in the selection of these parameters. In any case, it is obvious that the characteristics of the clad will in some instances limit the pressures at which the sintered powders may be worked. Accordingly, drastic increases in the working pressure will necessarily be preceded by adjustments to the jacket material composition or thickness. Representative working parameters for several materials are set forth in Table II.

TABLE II Material Temperature F.) Pressure (p.s.i.)

Example I Pure beryllium powders were hot pressed into the shape of a pressure vessel having a diameter of approximately six inches with a wall thickness of 0.250 inch. Pressing was effected in a helium atmosphere at 1450 F., utilizing a pressure of 10,000 p.s.i. The powder metallurgy product Was clad externally with a carbon steel jacket 0.75 inch thick, and pressurized internally with argon at 8000 p.s.i. while held at 1450 F.

Metallurgical analysis revealed a wrought structure, and a yield strength increase from approximately 50,000 p.s.i. to 75,000 p.s.i. was noted.

Example II Using a titanium base alloy of the nominal composition, by Weight, 13 percent vanadium, 11 percent chromium, 3 percent aluminum, balance titanium, the hot pressing parameters are 1500 F. at a pressure of 12,000 p.s.i. helium. For a pressure vessel of the same size, wall thickness and configuration as that in Example I, a Waspalloy (13.5 Co19.5 Cr4.25 Mo-1.25 Al3 Tibalance Ni) jacket of 0.5 inch thickness is utilized and the powders are worked at 10,000 p.s.i. in argon while held at 1750 F. to produce the desired wrought microstructure and a yield strength increase from 120,000 p.s.i. to approximately 180,000 p.s.i.

Example III Using a fine grain beryllium alloy powder, designated I 400 and having the nominal composition, by weight, 4.25 percent beryllia, .2 percent aluminum, .5 percent carbon, .3 percent iron, .1 percent magnesium, .15 percent silicon, balance beryllium, the hot pressing parameters are 1450 F. and 20,000 p.s.i. helium. Hot working at 15,000 p.s.i. and 1450 F. with inert gas against a carbon steel jacket of 0.75 inch thickness produces a wrought microstructure and a yield strength increase from 70,000 p.s.i. to approximately 120,000 p.s.i.

As may readily be seen, through the techniques taught in the present invention, pressure vessels are produced in a wide variety of shapes having optimum strength characteristics. Further, since there are no wall discontinuities, such as those arising from changes in microstructure at weld areas in more conventional articles, the reliability of such articles in the performance of their designed function is much increased.

While the invention has been described in connection with several examples and specific material compositions, its applicability to other systems will be evident within strength exceeding that of the sintered article at its recrystallization temperature, heating the clad article to the recrystallization temperature of the sintered article and applying a fluid pressure to the unclad wall of the article to effect deformation thereof against the clad in a hot working operation, and removing the clad from the article. 2. The method of forming a seamless pressure vessel by powder metallurgy techniques comprising the steps of: hot pressing metal powders into the shape of a pressure vessel in an inert atmosphere to form a fully dense sintered article,

cladding the sintered article to provide an exterior jacket therearound against which the sintered powders may be hot worked, the jacket at the recrystallization temperature of the sintered powders having a strength exceeding that of the sintered article and a modulus of elasticity less than that of the sintered article,

heating the jacketed article to effect recrystallization of the sintered powders and internally pressurizing the jacketed article with an inert fluid to compressively work the sintered powders against the jacket,

and removing at least the major portion of the jacket from the worked pressure vessel.

3. The method of claim 2 wherein the metal powders consist of as their principal component at least one metal selected from the group consisting of beryllium, titanium, aluminum, molybdenum and tungsten.

References Cited UNITED STATES PATENTS 2,337,247 12/ 1943 Kepler.

2,752,669 7/1956 Carr 29-421 X 2,916,809 12/1959 Schell 45-226 X 3,114,968 12/1963 Rudelick 29-4205 3,127,671 4/ 1964 Hayes 29-423 3,135,045 6/1964 Schane 29-423 3,156,973 11/1964 Lieberman. 3,197,851 8/1965 Aleck 29421 3,242,563 3/ 1966 Turner 29-423 3,249,410 5/1966 Lorenzo 75-214 X 3,268,368 8/1966 Mackiw 75-214 X OTHER REFERENCES Beryllium Processing by Powder Metallurgy: by H. G. Wihle and V. C. Potter, August 1961, Journal of Metals, pp. 537538.

BENJAMIN R. PADGETT, Primary Examiner. A. J. STEINER, Assistant Examiner.

US. Cl. X.R. 

